A cognitive network is a network comprising a large number of wireless devices in which different devices may use different radio access technologies (RATs), (e.g., cellular, wireless local area network (WLAN), Bluetooth, Zigbee, etc.), and operate over multiple unlicensed bands and/or TV white space channels. These wireless devices which are radio flexible and radio agile are referred as cognitive nodes.
To communicate to each other, nodes of a cognitive network discover neighbors and know which channel(s) may be used to communicate with each other. In conventional wireless networks a neighbor discovery mechanism uses a default frequency channel. For example, IEEE 802.11p wireless access in vehicular environments (WAVE) uses a single dedicated control channel which is used in neighbor discovery and association.
Both passive and active scanning may be used to search for an access point (AP). In passive scanning, each device scans individual channels to find a beacon signal. Periodically, APs broadcast a beacon, and the scanning device receives these beacons and takes note of the corresponding signal strengths. The beacons contain information about the AP, including service set identifier (SSID), supported data rates, regulatory class, etc. The scanning device may use this information along with the signal strength to compare APs and select one with which to associate.
In active scanning, the device initiates the process by sending a probe request frame, and APs within a range respond with a probe response frame. Active scanning enables a device to receive an immediate response from the APs without waiting for a periodic beacon transmission.
A cognitive node may scan a large amount of spectrum to find an operating channel. More specifically, Federal Communications Commission (FCC) has opened about 300 MHz of available spectrum ranging from 54-698 MHz, in a band referred to as TV White Space (TVWS). When a wireless device operating in this band is initially switched on, it may not have any knowledge about the operating channel and the bandwidth of the channel being used. An 802.11 device, for example, may operate in multiple channel spacing, (i.e., 5, 10, 20, and 40 MHz). The device may scan the channels with different channel spacing permutations to find the operating channel.
The FCC defined three devices that may operate in the TVWS band: a Mode I device, a Mode II device, and a sensing-only device. In addition, a TV bands database is used to keep track of which channels are available in a specific geographic location. TVWS-capable devices may query the TV bands database to determine a list of available channels in their location.
A Mode I device is a TVWS device that does not use an internal geo-location capability and access to the TV bands database to obtain a list of available channels. A Mode I device may obtain a list of available channels on which it may operate from either a fixed TVWS device or a Mode II device.
A Mode II device is a TVWS device that uses an internal geo-location capability and access to the TVWS database, either through a direct connection to the Internet or through an indirect connection to the Internet by way of a fixed TWWS device or another Mode II TVWS device, to obtain a list of available channels. A Mode II device may select a channel by itself and initiate and operate as part of a network of TVWS devices, transmitting to, and receiving from, one or more fixed TVWS devices or personal/portable TVWS devices. A Mode II device may provide its list of available channels to a Mode I device for operation by the Mode I device.
A sensing-only device is a TVWS device that uses spectrum sensing to determine a list of available channels.
In an 802.11 WLAN, the devices may operate under the regulatory class. Regulatory class for the orthogonal frequency division multiplex (OFDM) is an index into a set of values for radio equipment sets of rules. It includes the following parameters: the channel starting frequency, (which is a frequency used together with a channel number to calculate a channel center frequency), channel spacing, (which is the frequency difference between non-overlapping adjacent channel center frequencies), the channel set, (which is the list of integer channel numbers that are legal for a regulatory domain and class), the transmit power limit, (which is the maximum transmit power that is legal for a regulatory domain and class), and an emission limit set, (which is an enumerated list of spectral masks and emissions limits that are legal for a regulatory domain).